Cellulosic biomass has advantages in that it is abundant, renewable and inexpensive, and due to such advantages, the possibility of using the cellulosic biomass as a raw material for fuel alcohol such as ethanol or butanol is increasing. Cellulosic biomass comprises cellulose, hemicellulose and lignin as main components, which are strongly bonded to one another. Thus, in order to produce C-Sugar from the biomass in high yield, these three components need to be separated from one another by pretreatment.
Among the three components, cellulose has the properties of being thermally stable and being soluble in acid, and hemicellulose has the properties of being thermally unstable and being soluble in acid. On the other hand, lignin has the properties of being thermally stable and being soluble in alkali.
In order to efficiently separate these components from one another, which have different properties as described above, various pretreatment processes have been developed. Generally, a general pretreatment method is to selectively remove the hemicellulose and lignin components without loss of the cellulose component to thereby convert the cellulosic raw material into a form easy to enzymatically saccharify.
Major pretreatment methods include mechanical crushing, alkali swelling, dilute acid hydrolysis, hydrothermal pretreatment and steam explosion pretreatment methods, and combinations of such methods may also be used.
Among these methods, the hydrothermal pretreatment method can separate hemicellulose as a liquid by water under mild operating conditions while allowing cellulose and lignin to remain as a solid. It is a method that increases the recovery of hemicellulose having low thermal stability by the use of mild operating conditions.
Furthermore, in the steam explosion pretreatment method, a raw material is charged into a pressurized reactor and saturated stream is blown into the reactor to cause a pressurized reaction, and then pressure is suddenly discharged from the reactor, whereby an exploded raw material can be obtained. In this procedure, saccharification of hemicellulose may occur, or the structure of lignin may also be broken. This method has high energy efficiency, because the raw material does not need to be finely crushed.
Once the cellulose, hemicellulose and lignin components are separated from one another by the above-described pretreatment method, the cellulose component is subjected to an enzymatic saccharification step.
The enzyme that is involved in the saccharification is a catalyst having very high substrate selectivity, and the enzymatic reaction conditions are very mild so that the load in reactor design and operations is reduced. Cellulose saccharification is based on three actions. Endo-β-1,4-glucanase, exo-β-1,4-glucanase and β-glucosidase randomly attack cellulose to separate cellobiose from cellulose chain ends. When the antagonism between endo-β-1,4-glucanase and exo-β-1,4-glucanase is continued, the concentration of the cellobiose increases, and the activity of exo-β-1,4-glucanase is severely inhibited by accumulation of the cellobiose. The produced cellobiose is then cleaved to glucose by β-glucosidase. This cleavage procedure proceeds in a liquid phase. In addition, while the glucose is accumulated, β-glucosidase is also inhibited. Saccharification of the cellulose is influenced by all the three enzymes as described above.
However, it is well known that when fermentable sugar is produced from cellulosic biomass by pretreatment and saccharification processes, substances (acetic acid, HMF, furfural, etc.) capable of inhibiting fermentation are produced in the pretreatment process (degeneration occurs mainly in a high-temperature and high-pressure reaction), and particularly, when hydrothermal treatment is carried out, a large amount of a xylo-oligomer occurs after pretreatment to thereby inhibit enzymatic saccharification.
Korean Patent No. 1393412 discloses a method of producing a hydrolysate from cellulosic biomass by alkali soaking-steam pretreatment, which can recover useful substances. In the above patent document, alkali-based pretreatment rather than hydrothermal pretreatment is performed to thereby minimize the production of fermentation inhibitory substances by hydrothermal pretreatment, but there are disadvantages in that the separation process is complicated due to the use of the alkaline substance and in that the efficiency of saccharification and fermentation can be reduced if the alkaline substance is not removed to a suitable concentration or lower.
Korean Patent No. 1392736 discloses an integrated process for production of bioethanol, which includes lignocellulosic biomass pretreatment using nitric acid prepared from nitric oxide. In this patent document, biomass is pretreated with nitric acid so that the production of fermentation inhibitory substances by hydrothermal pretreatment is minimized. However, there are also disadvantages in that the process is complicated due to separation of the acidic substance and in that the efficiency of saccharification can be reduced if the acidic substance is not removed to a certain concentration or lower.
Korean Patent No. 10-1273218 discloses a steam-explosion pretreatment method. In the method disclosed in this patent document, steam is blown into a reactor to pressurize the reactor, and then pressurize is suddenly discharged from the reactor to cause explosion to thereby break the structure of a raw material after pretreatment. Thus, a chip-type raw material having a size of about 2 cm×2 cm×1 cm can be used without pulverization, and energy required for pulverization can be saved, unlike the case of other pretreatments, and a strong biomass structure can be broken to facilitate its contact with an enzyme, thus increasing the efficiency of saccharification. However, there are disadvantages in that, because a pressurization process should be used in the explosion step, the overall process is complicated and a large amount of energy is consumed.
Accordingly, the present inventors have made extensive efforts to solve the above-described problems, and as a result, have found that, when biomass is pretreated so as to be separated into a solid and a liquid and the separated solid and liquid are subjected to a saccharification process and a monomerization process, respectively, to produce glucose and xylose, fermentation inhibitory substances can be removed in a cost- and energy-effective process, thereby completing the present invention.